The quiescent state of peripheral naive T lymphocytes was thought to be due to the lack of activation signals. Recent studies, however, have shown that T cell quiescence is not by default but actively maintained by both cell extrinsic and intrinsic mechanisms, about which little is known at the transcriptional level. Forkhead box (FOX) proteins comprise a large family of transcription factors with diverse functions in development, aging and cancer. Recently we discovered that transcription factor Foxp1 exerts essential cell-intrinsic regulation of the quiescence of naive T cells, providing direct evidence that mature T cell quiescence is actively maintained. Meanwhile, our new preliminary studies also start to reveal that Foxp1 transcriptional network plays an important role in agonist- induced T cell activation. In this proposal, we will combine various approaches to identify functional targets and molecular pathways regulated by Foxp1 in controlling T cell quiescence, and to define the critical roles of Foxp1 in antigen-induced T cell activation and responses in vivo. Knowledge obtained from these studies will provide information for the design of new therapeutic strategies that would manipulate T cell activation status for the treatment of autoimmune and infectious diseases, and aid in vaccine development.